Conditions previously defined for chemically inducing mutations of human fibroblasts in vitro will be used to induce transformation toward malignancy. Variant fibroblasts that can form dense loci on plastic and grow in soft-agar medium will be isolated and characterized, especially with regard to their capacity for unlimited proliferation in vitro and for tumorigenicity in nude mice. Rates of transformation will be compared in strains of cells from control humans and from humans who have large cancer risk. Properties of human cells transformed in vitro and in vivo will be studied partly by characterizing hybrids formed by fusing them to normal human cells. Methylation of DNA from active and inactive X chromosomes will be compared. The two kinds of DNA will be isolated by removal of the mouse DNA from mouse-human hybrid cells that have only active or inactive human X chromosome. Restriction enzyme digests of the two kinds of X-chromosomes will be compared. Clones of chimaeric DNA molecules containing specific fragments of X-chromosomal DNA will be used as molecular "hooks" to isolate the homologus fragments from active and inactive X-DNA for purposes of comparison. BIBLIOGRAPHIC REFERENCES: DeMars, R., S.L. LeVan, B.L. Trend and L.B. Russell. Abnormal ornithine carbamoyltransferase in mice having the sparse-fur mutation. Proc. Nat. Acad. Sci. (U.S.A.) 73 (1976) 1693-1697. DeMars, R. Mutagenesis detection with human and other mammalian somatic cells. Journal of Environmental Pathology and Toxicology. In Press (July, 1977).